This invention relates to a circuit arrangement for operating a high pressure discharge lamp having, during operation, an electrode which is in a cathodic phase, comprising
input terminals for connection to a supply voltage source, PA1 output terminals for connecting the high pressure discharge lamp, and PA1 means, coupled to the input terminals, for supplying an alternating lamp current having successive periods of opposite polarity to the high pressure discharge lamp, the lamp current per period having a mean value Im.
Such a circuit arrangement is known from U.S. Pat. No. 5,608,294. In the known circuit the lamp current at each period is provided with a superposition of a current pulse in a latter part of each period. In the case where a high pressure discharge lamp is operated with an AC current, each electrode of the lamp alternatingly functions as a cathode and as an anode during successive periods of the lamp current. During these successive periods the electrode emitting the lamp current is said to be in the cathodic phase and the other electrode to be in the anodic phase respectively. Because the total amount of current through the lamp is increased at the end of each period of the lamp current by means of the current pulse, the temperature of the electrode is sufficiently raised to increase the stability of the discharge arc. Accordingly, flickering of the high pressure discharge lamp can be substantially suppressed. Due to its flickering suppression properties the circuit arrangement is in particular suitable for operating a high pressure discharge lamp in a projection system like a projection television apparatus.
Lamps operated with the known circuit arrangement unfortunately showed a continuous increase of the arc voltage over an operating time of several hundred hours, which voltage increase appeared to continue when the lamp was experimentally operated for several thousand hours. As a stable luminous output of the lamp which is fairly constant over the life of the lamp is of vital importance for use in a projection system, a continuous arc voltage increase forms a serious draw back in reaching a long lamp life.